1. Field of the Invention
The present invention relates to an image pickup apparatus and an image pickup method. In particular, the invention relates to an image pickup apparatus and an image pickup method for correcting optical distortions contained in image information obtained by image picking up via an optical lens, and conducting contour correction on the image information.
2. Description of the Related Art
In an image pickup apparatus that picks up an image of a subject through an optical lens of a silver film camera or a digital camera and acquires an image which represents the subject, distortion is caused around the acquired image due to refraction. This distortion is typically called optical distortion. The lens used in the image pickup apparatus is formed so as to cancel the optical distortion. In the case of a zoom lens, however, it is difficult to conduct correction at both the tele-end and the wide end with the same lens configuration, and large optical distortion is apt to occur. In a single-focus lens as well, lenses of expensive materials or an increase in lens in the configuration is needed to conduct correction. Thus it is difficult to obtain a thin inexpensive lens configuration, and so optical distortion remains. In this way, it is difficult to make the distortion characteristic of the lens used in the image pickup apparatus equal to 0% because of cost and size limitations. It is considered that about 1% is visually adequate.
In the case where an acquired image is recorded on a film as in a silver film camera, correction of the image after recording is impossible and the optical distortion depends upon the lens performance. On the other hand, in the case where an image is acquired in the form of digital data and recorded on a recording medium as in the digital camera, it is possible to correct the image by computation processing even after recording. In the field of digital camera, therefore, techniques concerning optical distortion correction have been proposed heretofore.
Here, optical distortion is classified into two kinds, i.e., “pincushion distortion” in which corner portions of an image extend outside as shown in FIG. 6A and “barrel distortion” in which conversely the corner portions contract as shown in FIG. 6B. It is generally known that the distortion quantity (displacement quantity) depends on the distance from the optical center in both types. In other words, if the displacement quantity is linear, merely compression or expansion would occur. As a matter of fact, however, the displacement quantity is nonlinear as shown in FIG. 6C. In the case of a positive displacement quantity, each pixel is displaced to a position moved from the original position so as to be further away from the center, resulting in the “pincushion distortion.” In the case of a negative displacement quantity, each pixel is displaced to a position moved from the original position so as to be nearer the center, resulting in the “barrel distortion.”
As a conventional technique for correcting such optical distortion, there is a method for obtaining the correction quantity at coordinate on an image, storing the correction quantities in advance in a memory, and conducting linear interpolation on the basis of the stored correction quantities to correct respective pixels (for example, see Japanese Patent Application Laid-Open (JP-A) No. 6-292207). In this technique, however, the memory for storing the table of the correction quantities needs to have a storage capacity that depends upon the image size. As the image size becomes large, the storage capacity needed in the memory also becomes large, resulting in a reduction in the area available for work and an increase in price.
A technique contrived to solve the problem is a method of representing the correction quantity by using an approximation and conducting correction. In other words, it is generally known that the displacement quantity of the optical distortion can be approximately represented using a polynomial. The reciprocal of this polynomial is used as a correction formula. In this technique, it is not necessary to retain the correction quantities at respective coordinates in a table. So long as the parameters (coefficients of the polynomial) are stored in the memory, all coordinates of an image before and after correction can be associated by computation (for example, see JP-A No. 11-252431). There is also a technique of storing different parameters for every focal distance, selecting the parameters according to the focal distance of the image taken, and conducting a correction in order to cope with different focal distances at the tele-end and the wide end (for example, see JP-A No. 11-275391). In addition, there is also a technique of increasing the speed of the correction processing by converting digital image data (RGB data) to YUV data, thinning the UV data in the YUV data, and then conducting a correction (for example, see JP-A No. 11-250240).
Application software executed in a computer (PC) includes application software for conducting optical distortion correction processing. Therefore, it is also possible to correct the optical distortion by transferring image data picked up by a digital image pickup apparatus to a PC and executing the application software on the PC side. However, effort is required for installing application software in the PC to conduct the optical distortion correction processing and effort is required for transferring digital image data to be corrected onto the PC (i.e., into the RAM incorporated in the PC). For reducing the work burden imposed on photographers, therefore, it is desirable to conduct the correction on the digital image pickup apparatus, as in the above-described techniques.
In the above-described distortion correction, correction is conducted for optical distortion so as to obtain an image having a uniform distortion factor over the whole picture. The optical distortion becomes larger as the position moves from the center portion of the optical lens toward the peripheral portions. In most techniques for correcting the optical distortion therefore, including the above-described techniques, a coordinate transform for making the movement quantity larger as the position moves from the center portion of an image indicated by image data obtained via the optical lens toward the peripheral portion (interpolation computation based on several nearby points) is conducted. This results in a problem that the change of frequency response becomes notable on moving from the central portion toward the peripheral portion of the image according to the image data after correction and consequently the picture quality is degraded.
For example, when the coordinate transform involves pixel interpolation, the movement quantity of a pixel GH (on FIG. 7), which is obtained by the pixel interpolation, based on the optical distortion becomes larger as the point moves from the center portion of the image toward the peripheral portion. This problem occurs regardless of whether the optical distortion is pincushion distortion or barrel distortion.